A difficulty with the use of aluminum alloy products in corrosive environments, such as heat exchanger tubing, is pitting corrosion. Once small pits start to form, corrosion actively concentrates in the region of the pits, so that perforation and failure of the alloy occurs much more rapidly than it would if the corrosion were more general.
Pitting corrosion is accelerated when there is a strong tendency towards surface passivation due to the growth of a corrosion resistant oxide film over the vast majority of the tube surface. However, such a film is never 100% intact due to the presence of discontinuities in the oxide, which are in some instances due to nonmetallic inclusions or intermetallics in the metal. With such a situation the passive areas are cathodic to any corrosion which may begin at the discontinuities. With such a large cathode/anode area ratio, the dissolution rate at the active sites is very rapid and the tube will perforate by pitting in 2-6 days in the SWAAT test.
Anthony et al., U.S. Pat. No. 3,878,871, issued Apr. 22, 1975, describes a corrosion resistant aluminum alloy composite material comprising an aluminum alloy core containing from 0.1 to 0.8% manganese and from 0.05 to 0.5% silicon, and a layer of cladding material which is an aluminum alloy containing 0.8 to 1.2% manganese and 0.1 to 0.4% zinc.
Sircar, WO 97/46726, published Dec. 11, 1997 describes a corrosion resistant AA 3000 series aluminum alloy containing controlled amounts of copper, zinc and titanium. It has a titanium content of 0.03 to 0.30%, but this level of titanium raises the pressures required for extrusion, which will ultimately lower productivity.
It is an object of the present invention to develop an aluminum alloy which can combine both strength and resistance to pitting corrosion. It is a further object of the invention to provide an aluminum alloy which is resistant to pitting corrosion and which is particularly useful in the production of extruded products, such as heat exchanger tubing.
It is yet another object of the invention to provide an aluminum alloy resistant to pitting corrosion which is useful in the production of sheet or plate products.